ABSTRACT Post-transcriptional modification of RNA is a fundamental and essential aspect of gene expression, with important consequences for many aspects of human health and disease. One pathway for RNA modification is guided by small nucleolar RNAs (snoRNAs). SnoRNAs are non-coding and primarily direct site-specific modifications on ribosomal RNA (rRNA). However, some snoRNAs have ?non-canonical? biological effects that are independent of rRNA modification. These non-canonical snoRNA effects contribute to cancer, neurobiology, metabolism, and oxidative stress, via diverse mechanisms such as direct protein binding, generation of microRNAs, and regulation of messenger RNA (mRNA) splicing. Our work has shown that four snoRNAs encoded by the Rpl13a genetic locus are non-canonical regulators of reactive oxygen species (ROS) and oxidative stress. Loss of these snoRNAs protects cells and animals from many kinds of insults, including metabolic stress, sepsis, and diabetes. However, the precise molecular mechanism has been elusive. Our preliminary data suggest that a mechanism for this effect is snoRNA-guided methylation of mRNA, affecting both mRNA stability and protein production. This would be a novel function for snoRNAs. The objective of this application is therefore to determine whether snoRNA-guided methylation of mRNA links the Rpl13a snoRNAs to ROS and oxidative stress. Our central hypothesis is that the Rpl13a snoRNAs guide the enzyme fibrillarin to catalyze 2?-O-methylation of select mRNA targets, and that these methylations alter mRNA stability and translation. Aim 1 will verify a putative mRNA target of Rpl13a snoRNA-guided methylation. Aim 2 will define the role of this mRNA modification in cellular physiology, including ROS production and oxidative stress. Aim 3 will identify novel targets of snoRNA-guided methylation, using methods to capture snoRNA-mRNA interactions, and by mapping these modifications transcriptome-wide. The overall impact of this project will be to define a molecular mechanism linking the Rpl13a snoRNAs to ROS and oxidative stress, while at the same time expanding the known functions of snoRNAs to include 2?OMe modification of mRNA.